U.S. patent number 6,824,328 [Application Number 09/549,902] was granted by the patent office on 2004-11-30 for vapor collection and treatment of off-gas from an in-situ thermal desorption soil remediation.
This patent grant is currently assigned to Board of Regents, The University of Texas System. Invention is credited to George L. Stegemeier, Harold J. Vinegar.
United States Patent |
6,824,328 |
Vinegar , et al. |
November 30, 2004 |
Vapor collection and treatment of off-gas from an in-situ thermal
desorption soil remediation
Abstract
An in situ thermal desorption system may be used to remove
contamination from soil. Off-gas removed from the soil may be
transported from the soil to a treatment facility by high
temperature hoses and plastic piping. The use of high temperature
hose and plastic pipe may reduce the capital cost, installation
cost, and operating cost as compared to conventional transport
systems from thermal desorption soil remediation systems. The high
temperature hose and plastic pipe are highly resistant to corrosion
caused by the off-gas. The treatment facility may separate the
off-gas into a liquid stream and a vapor stream. The liquid stream
and the vapor stream may be processed to reduce contaminants within
the liquid stream and vapor stream to acceptable levels.
Inventors: |
Vinegar; Harold J. (Houston,
TX), Stegemeier; George L. (Houston, TX) |
Assignee: |
Board of Regents, The University of
Texas System (Austin, TX)
|
Family
ID: |
33452568 |
Appl.
No.: |
09/549,902 |
Filed: |
April 14, 2000 |
Current U.S.
Class: |
405/128.4;
166/60; 405/128.6 |
Current CPC
Class: |
B09C
1/062 (20130101); H05B 3/03 (20130101); H05B
3/0004 (20130101); E21B 36/04 (20130101); B09C
2101/00 (20130101) |
Current International
Class: |
B09C
1/00 (20060101); B09C 1/06 (20060101); E21B
36/04 (20060101); E21B 36/00 (20060101); H05B
3/00 (20060101); H05B 3/02 (20060101); H05B
3/03 (20060101); B09B 001/06 () |
Field of
Search: |
;405/128.1,128.15,128.2,128.35,128.4,128.6 ;166/75.13,60 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
US Army Corps of Engineers Engineering and Design "Liquid Process
Piping" 1999.* .
US Army Corps of Engineers Engineering and Design "Multi-Phase
Extraction" 1999, chapter 5.* .
Vinegar et al., "In Situ Thermal Desorption of Soils Impacted with
Chlorinated Solvents," Jun. 1999, 23 pgs. .
Vinegar et al., "In Situ Thermal Desorption using Thermal Wells and
Blankets," May 1998, 1 pg. .
Conley et al., "In Situ Thermal Desorption of Refined Petroleum
Hydrocarbons from Saturated Soil," May 2000, pp. 1-10. .
Hansen et al., "In Situ Thermal Desorption of Coal Tar," Dec. 1998,
pp. 1-22. .
Vinegar et al., "Remediation of Deep Soil Contaimination using
Thermal Vacuum Wells," .COPYRGT. 1997 Society of Petroleum
Engineers, pp. 905-918..
|
Primary Examiner: Kreck; John
Attorney, Agent or Firm: Meyertons, Hood, Kivlin, Kowert
& Goetzel, P.C. Meyertons; Eric B.
Claims
What is claimed is:
1. A thermal soil remediation system for removing contaminants from
soil, comprising: a plurality of strip heaters in the soil; a
plurality of vacuum wells comprising perforated casings; high
temperature hoses configured to couple the perforated casings to a
vacuum manifold that comprises plastic piping, wherein the high
temperature hoses convey heated off-gas removed from the soil to
the vacuum manifold; and wherein one of the strip heaters comprises
a first heater section and a second heater section, and wherein a
cross-sectional area of the first heater section is less than a
cross-sectional area of the second heater section so that the first
heater section generates more heat than the second heater section
during use, and wherein at least the first heater section has a
substantially rectangular cross section.
2. The system of claim 1, wherein a material of construction of the
high temperature hoses and a material of construction of the vacuum
manifold inhibits corrosion by decomposition products of
contaminants that are generated by thermal decomposition from heat
provided by the strip heaters.
3. The system of claim 1, wherein a material of construction of the
high temperature hoses and a material of construction of the vacuum
manifold inhibits corrosion by HCl (hydrogen chloride).
4. The system of claim 1, wherein a casing of a vacuum well is
positioned between legs of a strip heater.
5. The system of claim 1, wherein the strip heater having the first
heater section and the second heater section is formed in a U
shape, and wherein the first heater section is located at the
bottom of the U-shaped strip heater.
6. The system of claim 1, wherein at least one of the strip heaters
is positioned in a trench formed in the soil.
7. The system of claim 1, wherein at least one of the hoses
comprises reinforcement to inhibit collapse of the hose when vacuum
is drawn on the soil through the hose.
8. The system of claim 1, wherein the vacuum manifold comprises
chlorinated polyvinyl chloride piping.
9. The system of claim 1, wherein lengths of the hoses provide
residence times in the hoses for hot off-gas flowing in the hoses,
and wherein the residence times allow the vapor to cool at least to
temperatures that will inhibit temperature of the vacuum manifold
from exceeding an upper working temperature limit of the vacuum
manifold.
10. The system of claim 1, further comprising a treatment facility
coupled to the vacuum manifold, wherein the treatment facility is
configured to reduce contamination in the off-gas to an acceptable
level.
11. A thermal soil remediation system for removing contaminants
from soil, comprising: a plurality of strip heaters in the soil; a
plurality of vacuum wells comprising perforated casings; high
temperature hoses configured to couple the perforated casings to a
vacuum manifold that comprises plastic piping, wherein the high
temperature hoses convey heated off-gas removed from the soil to
the vacuum manifold; and wherein at least one of the strip heaters
comprises a U-shaped strip, and wherein a cross-sectional shape of
the at least one strip heater is rectangular.
12. The system of claim 11, further comprising one or more
insulated centralizers, wherein the one or more insulated
centralizers are positioned between the perforated casings and the
strip heaters, and wherein the perforated casings are positioned
between legs of the strip heaters.
13. The system of claim 11, wherein the perforated casings are
configured such that the vacuum wells draw vapor into the
perforated casings, and wherein the vapor is removed as off-gas
from the soil.
14. The system of claim 11, further comprising an impermeable
barrier, wherein the impermeable barrier is configured to limit an
amount of air drawn into the soil.
15. The system of claim 14, further comprising a thermal barrier,
wherein the thermal barrier is configured to inhibit thermal
degradation of the impermeable barrier.
16. The system of claim 11, further comprising a thermal
barrier.
17. The system of claim 11, further comprising an external casing,
wherein the external casing is configured to inhibit current
leakage in soil.
18. The system of claim 11, further comprising: an external casing
for a strip heater of the strip heaters; and fill material, wherein
the fill material is positioned adjacent to the external casing and
the strip heater.
19. The system of claim 18, wherein the fill material is configured
to promote thermal degradation of the contaminants in the soil.
20. The system of claim 11, wherein a material of construction of
the high temperature hoses and a material of construction of the
vacuum manifold inhibits corrosion by decomposition products of
contaminants that are generated by thermal decomposition from heat
provided by the strip heaters.
21. The system of claim 11, wherein at least one strip heater is
positioned in the soil by vibrating the strip heater into the
soil.
22. The system of claim 11, wherein a strip heater of the strip
heaters comprises a first heater section and a second heater
section, and wherein a cross-sectional area of the first heater
section is less than a cross-sectional area of the second heater
section so that the first heater section generates more heat than
the second heater section during use.
23. The system of claim 11, further comprising a treatment facility
coupled to the vacuum manifold, wherein the treatment facility is
configured to reduce contamination within the off-gas to an
acceptable level.
24. A thermal soil remediation system for removing contaminants
from soil, comprising: strip heaters, wherein at least a portion of
each strip heater is positioned in the soil; vacuum wells
comprising perforated casings; high temperature hoses configured to
couple the perforated casings to a vacuum manifold that comprises
plastic piping, wherein the high temperature hoses convey heated
off-gas removed from the soil to the vacuum manifold, and wherein
lengths of the hoses provide residence times for hot off-gas
flowing in the hoses, and wherein the residence times allow the
vapor to cool at least to temperatures that will inhibit
temperature of the vacuum manifold from exceeding an upper working
temperature limit of the vacuum manifold; and insulated
centralizers positioned between the perforated casings and the
strip heaters, and wherein the perforated casings are positioned
between legs of the strip heaters; wherein one or more of the strip
heaters comprise a first heater section and a second heater
section, and wherein a cross-sectional area of the first heater
section is less than a cross-sectional area of the second heater
section so that first heater section generates more heat than the
second heater section during use.
25. The system of claim 24, wherein the vacuum wells are configured
to draw vapor into the perforated casings, and wherein the vapor is
removed as off-gas from the soil.
26. The system of claim 24, further comprising a thermocouple
positioned in one of the perforated casings.
27. The system of claim 24, further comprising a barrier that is
substantially impermeable to air and water, and wherein the barrier
is positioned at or near a soil-air interface.
28. The system of claim 24, further comprising a thermal barrier
positioned near or at a soil-air interface.
29. The system of claim 24, further comprising a first barrier
substantially impermeable to air and water, a second barrier which
is a thermal barrier, wherein the second barrier is configured to
inhibit vapor from condensing on the first barrier, and wherein the
first barrier and the second barrier are positioned near or at a
soil-air interface.
30. The system of claim 24, further comprising a treatment facility
coupled to the vacuum manifold, wherein the treatment facility is
configured to reduce contamination in the off-gas to an acceptable
level.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to soil remediation, and
more particularly to a vapor collection system and treatment
facility for off-gas from an in situ thermal desorption soil
remediation process.
2. Description of Related Art
Contamination of subsurface soils has become a matter of great
concern in many locations. Subsurface soil may become contaminated
with chemical, biological, and/or radioactive contaminants.
Contamination of subsurface soil may occur in a variety of ways.
Hazardous material spills, leaking storage vessels, and landfill
seepage of improperly disposed of materials are just a few examples
of the many ways in which soil may become contaminated.
Contaminants in subsurface soil can become public health hazards if
the contaminants migrate into aquifers, into air, or into the food
supply. Contaminants in subsurface soil may migrate into the food
supply through bio-accumulation in various species that are part of
the food chain.
There are many methods for removal of contaminants from subsurface
soil. Some possible methods for treating contaminated subsurface
soil include excavation followed by incineration, in situ
vitrification, biological treatment, and in situ chemical
treatment. Although these methods may be successfully applied in
some applications, the methods can be very expensive. The methods
may not be practical if many tons of soil must be treated.
One process that may be used to remove contaminants from subsurface
soil is a soil vapor extraction (SVE) process. A SVE process
applies a vacuum to a well to draw air through subsurface soil. The
air carries volatile contaminants towards the source of the vacuum.
Off-gas removed from the soil by the vacuum may include
contaminants that were within the soil. The off-gas may be
transported to a treatment facility. The off-gas removed from the
soil may be processed in the treatment facility to reduce
contaminants within the off-gas to acceptable levels.
The permeability of the subsurface soil may limit the effectiveness
of a SVE process. Air and vapor may flow through subsurface soil
primarily through high permeability regions of the soil. The air
and vapor may bypass low permeability regions of the soil. Air and
vapor bypassing of low permeability regions may allow large amounts
of contaminants to remain in the soil after a SVE process has
treated the soil. Reduced air permeability due to water retention,
stratified soil layers, and heterogeneities within the soil may
cause regions of high and low permeability within subsurface
soil.
Reduced air permeability due to water retention may inhibit contact
of the flowing air with the contaminants in the soil. A partial
solution to the problem of water retention is to dewater the soil.
The soil may be dewatered by lowering the water table and/or by
using a vacuum dewatering technique. These methods may not be
effective methods of opening the pores of the soil to admit
airflow. Capillary forces may inhibit removal of water from the
soil when the water table is lowered. Lowering the water table may
result in moist soil. Air conductivity through moist soil is
limited.
A vacuum dewatering technique may have practical limitations. The
vacuum generated during a vacuum dewatering technique may diminish
rapidly with distance from the dewatering wells. The use of a
vacuum dewatering technique may not result in a significant
improvement to the soil water retention problem. The use of a
vacuum dewatering technique may result in the formation of
preferential passageways for air conductivity located adjacent to
the dewatering wells.
Many types of soil are characterized by horizontal layering with
alternating layers of high and low permeability. A common example
of a layered type of soil is lacustrine sediments. Thin beds of
alternating silty and sandy layers characterize lacustrine
sediments. If an SVE well intercepts several such layers, nearly
all of the induced airflow occurs within the sandy layers and
bypasses the silty layers.
Heterogeneities may be present in subsurface soil. Air and vapor
may preferentially flow through certain regions of heterogeneous
soil. Air and vapor may be impeded from flowing through other
regions of heterogeneous soil. For example, air and vapor tend to
flow preferentially through voids in poorly compacted fill
material. Air and vapor may be impeded from flowing through overly
compacted fill material. Buried debris within fill material may
also impede the flow of air and vapor through subsurface soil.
In situ thermal desorption (ISTD) may be used to increase the
effectiveness of a SVE process. An ISTD soil remediation process
involves in situ heating of the contaminated soil to raise the
temperature of the soil while simultaneously removing off-gas by
vacuum. In situ heating may be preferred over convective heating by
the introducing of a hot fluid (such as steam) into the soil
because thermal conduction through soil is very uniform as compared
to mass transfer through soil. Thermal conductivity of an average
soil may vary by a factor of about two throughout the soil. Fluid
flow conductivity of an average soil may vary by a factor of
10.sup.8 throughout the soil.
Soil may be heated by radiant heating in combination with thermal
conduction, by radiant by radio frequency heating, or by electrical
formation conduction heating. Conductive heating may be a preferred
method of heating the soil because conductive heating is not
limited by the amount of water present in the soil. For soil
contamination within about 2 feet of the soil surface, thermal
blankets may apply conductive heat to the soil. For deeper soil
contamination, heaters placed in wells may apply conductive heat to
the soil. Coincident or separate source vacuum may be applied to
remove vapors from the soil. U.S. Pat. No. 4,984,594 issued to
Vinegar et al, which is incorporated by reference as if fully set
forth herein, describes an ISTD process for soil remediation of low
depth soil contamination. U.S. Pat. No. 5,318,116 issued to Vinegar
et al., which is incorporated by reference as if fully set forth
herein, describes an ISTD process for treating contaminated
subsurface soil with conductive heating.
A conductive heat ISTD soil remediation process may have several
advantages over a simple soil vapor extraction system. The heat
added to the contaminated soil may raise the temperature of the
soil above the vaporization temperatures of the soil contaminants.
If the soil temperature exceeds the vaporization temperature of a
soil contaminant, the contaminant will become a vapor. The vacuum
may be able to draw the vaporized contaminant out of the soil. Even
heating the soil to a temperature below the vaporization
temperature of the contaminants may have beneficial effects.
Increasing the soil temperature will increase the vapor pressure of
the contaminants in the soil and allow an air stream to remove a
greater portion of the contaminants from the soil than is possible
at lower soil temperatures.
Most soil formations include a large amount of liquid water as
compared to contaminants. Raising the temperature of the soil to
the vaporization temperature of the water will boil the water. The
resulting water vapor may volatize contaminants within the soil by
steam distillation. An applied vacuum may then remove the volatized
contaminants and water vapor from the soil. Steam distillation
within the soil may result in the removal of medium and high
boiling point contaminants from the soil.
In addition to allowing greater removal of contaminants from the
soil, the increased heat of the soil may result in the destruction
of contaminants in situ. The presence of an oxidizer, such as air,
may result in the oxidation of the contaminants that pass through
soil that is heated to high temperatures. Contaminants within the
soil may be altered by pyrolysis to form volatile compounds that
are removed from the soil by the vacuum.
Heating the subsurface soil may result in an increase in the
permeability of the soil. A visible indication of the increase in
permeability of soil may be seen in the surface of dry lake beds.
As a lake bed dries, the soil forms a polygonal network of wide
cracks. In subsurface soil, the creation of a network of cracks may
result in enhanced vacuum driven transport within the soil.
Laboratory measurements also indicate that the microscopic
permeability of a dry mud is substantially greater than the
permeability of the original mud. The macroscopic and microscopic
increase in permeability of dried soil allows an ISTD soil
remediation process to be applied to low permeability clays and
silts that are not amenable to standard soil vapor extraction
processes.
A typical ISTD soil remediation process may include four major
components. The components may be heaters, off-gas collection
piping, an off-gas treatment system, and instrumentation and power
control systems.
For shallow contaminated soil, the heat may be applied to the soil
by a heating blanket placed on top of the soil. Shallow
contaminated soil includes soil contamination that does not extend
below a depth of about 3 feet. For deeper contaminated soil, heat
may be applied to the soil by heater wells.
The heat may be applied by a combination of radiant transfer and
heat conduction. The heater element radiantly heats a casing, and
the casing conductively heats the soil. The heating element of a
heater well may be constructed from two NICHROME.RTM. wire loops.
Interlocking ceramic beads may be positioned on the wire loops. The
heating element may be supported on either side of a 310 stainless
steel strip by small stainless steel bolts. The strip may be
suspended from a carbon steel top hat inside a 3.5-inch stainless
steel casing. The casing maybe sanded into a 6-inch augered hole.
The casing may include a welded top flange that seals to a silicone
rubber vapor barrier placed on top of the soil. Four thermocouples
may be attached to the NICHROME.RTM. heating element for
temperature control. A heater well may cost approximately $180 per
foot to produce. The heater well may require an installation time
of about 6 man hours.
In addition to the components of a heater well, a heater/suction
well includes an outer 4.5-inch stainless steel screened liner and
a flange above the surface flange. The additional flange connects
to a vacuum manifold. A heater/suction well may cost about $240 per
foot to produce. The heater/suction well may require an
installation time of about 8 man hours.
A ratio of heater wells to heater/suction wells may be used during
an ISTD soil remediation process. For example, an alternating
pattern of heater wells and heater/suction wells may be used in a
soil remediation system. Alternately, an ISTD soil remediation
process may use only heater/suction wells. After remediation is
complete, the wells may be pulled out of the ground with a crane.
The holes may then be sealed by grouting to the surface. Often the
condition of the wells after removal is poor. The wells may be
corroded and/or bent. Extensive rework may be required to bring a
well to a condition where it can be used again in another ISTD soil
remediation process.
The off-gas collection piping may connect an array of suction wells
to an off-gas treatment facility. The off-gas collection piping may
include a plurality of metal, interconnected pipes. The
interconnected piping may be flanged piping that requires careful
alignment during installation. A crane may be used to lift and
position the piping. The piping may be insulated piping that
includes internal electric heaters. The insulation and the heaters
prevent condensation of the vapor in the piping. The internal
electric heaters require extra power supplies, wiring, and control
units. Setting up the vapor collection piping constitutes a large
part of the field installation cost of an ISTD soil remediation
process.
A high soil temperature may destroy most of the soil contaminants
before the contaminants are drawn to the surface facilities. A
flameless thermal oxidizer may treat remaining contaminants within
the off-gas stream. One commercial ISTD soil remediation system
uses an 1800 scfm regenerative thermal oxidizer manufactured by
Thermatrix Inc. of San Jose, Calif. The Thermatrix 1800 thermal
oxidizer utilizes a ceramic media matrix to establish a stable and
efficient reaction zone with an operating temperature range of
1800-1900.degree. F. The Thermatrix 1800 includes about 65,000
pounds of ceramic matrix that has a high thermal inertia. A saddle
type geometry of the ceramic matrix promotes efficient mixing. The
Thermatrix 1800 thermal oxidizer has a guaranteed destruction
efficiency for chlorinated organic compounds of 99.99+%.
During initial startup, the thermal oxidizer may be preheated with
a gas burner until a desired temperature profile is created. The
burner is then turned off and the temperature profile inside the
thermal oxidizer is maintained by addition of fuel (propane) that
is mixed with air at ambient temperature. Once a stable profile is
obtained, the vapor stream is allowed to enter the oxidizer. Fuel
may be added or withheld from the thermal oxidizer to maintain a
substantially stable temperature profile within the thermal
oxidizer. Gases leaving the thermal oxidizer may be cooled in a
heat exchanger. The gases may then be passed through a carbon
absorption bed for backup and polishing.
Thermal oxidizers are costly to purchase, set up, and operate. The
capital expense of a vapor treatment system described above is very
high (more than one million dollars). Thermal oxidizers may be
large and heavy units that are expensive to mobilize. For example,
the Thermatrix 1800 thermal oxidizer has an on-site footprint of
about 52 feet by 8 feet. The unit has 65,000 pounds of ceramic
saddles. It must be transported to the site on a separate
double-drop trailer. The transportation cost to and from a soil
remediation site may be $70,000 or more. A thermal oxidizer
requires continuous manned operation. The thermal oxidizer unit is
the principal reason for manned operation of an ISTD soil
remediation process. manned operation. The thermal oxidizer unit is
the principal reason for manned operation of an ISTD soil
remediation process.
An ISTD soil remediation process may require a large amount of
computerized instrumentation for thermal well control and
temperature monitoring. A well controller may be used to control a
pair of thermal wells. Each well controller may monitor heater
thermocouples and control power applied to a pair of thermal wells.
The well controllers may be electrically connected to a central
computer over a field wide data link. Each well controller may cost
about $800. Thermocouples and control wiring for the thermal wells
are extensive and laborious to install, connect, and troubleshoot.
Thermocouples may be driven into the soil at various locations in a
region undergoing an ISTD soil remediation process to allow for
temperature monitoring. The thermocouples may be polled by selected
well controllers.
Well controllers enable the heater wells to apply heat to the soil
at a higher rate than a steady state heat injection rate. Although
a high rate can be applied at the beginning of the remediation
process, the well controllers must lower the heating rate to
prevent metallurgical damage to the heater wells. Thus, there may
only be a small net acceleration of the heating process due to
heating rate control. Moreover, the well controllers increase the
chance of heater failure because they are controlling temperature
at a single thermocouple location. If the thermocouple is not
located at the hottest portion of a heater, the hottest portion of
the heater may be maintained at an excessively hot temperature that
could cause the heating element to fail.
The on-site equipment may include three trailers. The three
trailers may be a process trailer, a control trailer, and an
electrical trailer. The process trailer, which may contain the
thermal oxidizer, heat exchangers, carbon beds and a vacuum source,
may occupy approximately an 8-foot by 52-foot area. The control
trailer, which contains all of the instrumentation and programming
for the ISTD soil remediation system, may occupy approximately an
8-foot by 48-foot area. The electrical trailer, which provides
power to the system, may occupy approximately an 8.5-foot by
48-foot area. occupy approximately an 8-foot by 48-foot area. The
electrical trailer, which provides power to the system, may occupy
approximately an 8.5-foot by 48-foot area.
SUMMARY OF THE INVENTION
An ISTD soil remediation process may be used to treat a region of
contaminated soil. Conductive heat may be applied to the soil by a
plurality of strip heaters. For low depth soil contamination, the
strip heaters may be placed in trenches within the contaminated
soil. For deeper soil contamination, the strip heaters may be
vertically positioned in heater wells, or in combined heater and
suction wells spaced throughout the contaminated soil. Vacuum
sources that are coincident to or separate from the strip heaters
may be applied to the soil to remove off-gas from the soil.
A strip heater may include a heater section, transition sections,
and cold pins. The heater section may be formed of a high
temperature, chemical resistant metal. The heater section
dissipates heat when the strip heater is connected to a power
source. The metal that forms the heater section may be, but is not
limited to, stainless steel, incoloy.RTM., or nichrome.RTM.. The
specific metal used to form the heater section of a strip heater
may be chosen based on cost, the operative temperature of the soil
remediation process, the electrical properties of the metal, the
physical properties of the metal, and the chemical resistance
properties of the metal.
A heater section may have a large cross section area as compared to
a cross sectional area of a conventional heater element. The large
cross sectional area of the heater section may result in a smaller
electrical resistance for the strip heater as compared to
conventional heaters of equivalent length. The smaller electrical
resistance allows several strip heaters to be connected in series.
The ability to connect several strip heaters in series greatly
simplifies the wiring requirements for an ISTD soil remediation
system. The large cross sectional area of the heater section also
allows a large contact area between the heater section and material
placed adjacent to the heater section. The large contact area may
promote dissipation of heat produced in the strip heater into
surrounding soil. The heat is applied to the soil by conduction.
Compared to conventional radiant heating, a heater strip may
operate at a lower temperature for the same power input. Avoiding
radiant energy transfer improves the reliability of the heating
system.
A heater section of a strip heater may be formed with a rectangular
cross sectional shape. For example, the heater section may be a
1-inch by 1/8-inch strip of stainless steel. A heater strip may be
40-feet or more in length. Strip heaters having other cross
sectional shapes may also be used. A strip heater may be formed
with a variable cross sectional area so that greater heat
dissipation occurs at certain portions of the strip heater
(sections having a smaller cross sectional area) than at other
portions of the strip heater. A local high heat dissipation section
of a strip may be positioned adjacent to soil that requires extra
heat dissipation, such as wet soil or the top and bottom sections
to counteract heat loss. A strip heater may be formed with sections
that have a large cross sectional area. A large cross sectional
area section of a strip heater may be placed adjacent to an
impermeable section of soil that does not need to be heated by the
strip heater. The cross sectional area of sections of a strip
heater may be less at the top and bottom of the heater strip so
that the strip heater diffuses more energy at the top and bottom of
the strip heater.
Transition sections may be welded to each end of a heater section
of a strip heater. Pins may be welded to the transition sections.
For example, the transition sections may be 6-inch long strips of
1-inch by 1/2-inch stainless steel that are welded to the ends of a
1-inch by 1/8-inch 20-foot long heater section. The pins may be
3/8-inch nickel pins. The pins may extend above the soil surface
when the strip heater is inserted into the soil. A mechanical
Kerney lug may be used to splice the nickel pins to copper cable.
The copper cable may be electrically coupled to a power source,
such as a transformer. Long nickel strips may be attached to a
heater section to form long unheated sections of a strip heater.
Long unheated sections of a strip heater may be needed for deep
soil contamination that is not near the soil surface.
A strip heater that will be used to treat deep soil contamination
may be bent into a U shape. The strip heater may be placed into an
augered hole. The hole may be packed with sand, gravel, or with
larger sized fill material. The fill material may push legs of the
strip heater against a wall of the hole. Larger sized fill material
may promote off-gas flow through the fill material. The fill
material may acts as a thermal transfer agent between the strip
heater and the soil. The fill material may include catalyst
material, such as alumina, that enhances the thermal breakdown of
contaminants. A suction well may be formed by inserting a
perforated casing between legs of the strip heater. Attaching the
perforated casing to a vacuum source allows vacuum to remove vapor
from the soil as off-gas. Positioning the casing between legs of a
U-shaped strip heater allows the off-gas to pass through a high
temperature zone before being removed from the soil. Passing the
off-gas through the high temperature zone may result in the thermal
degradation of contaminants within the off-gas.
As an alternative to placing a strip heater in an augered hole, the
strip heater may be driven into the soil. A drive rod may be
positioned at the center of a strip heater. The drive rod may then
be pounded into the soil. When the end of the strip heater is at
the correct depth, the drive rod may be withdrawn. The drive rod
does not need to be a continuous rod. The drive rod may be made of
threaded sections that are assembled together as the drive rod is
pounded deeper into the soil. A geoprobe or a cone penetrometer rig
may be used to drive the heater element into the soil. Also, a
sonic rig could be used to vibrate a strip heater to a desired
depth. The area between the legs of the strip heater may be filled
with fill material and/or a perforated casing. The perforated
casing may be attached to a vacuum source to form a suction well.
The fill material may include catalyst material that enhances
thermal breakdown of contaminants.
Driving or vibrating a heater strip into the soil may eliminate
problems associated with disposing of cuttings formed during the
formation of an augered hole. Avoidance of the production of
cuttings may be particularly advantageous at extremely toxic or
radioactive sites. Also, driving or vibrating a strip heater into
the soil advantageously places a portion of the strip heater in
direct contact with the soil to be heated.
Strip heaters may be placed horizontally in contaminated soil.
Horizontally oriented strip heaters may be especially useful for
treating soil contamination that extends less than about 4 feet
under the soil surface. Horizontally oriented strip heaters may be
placed in trenches. The trenches may be formed in the contaminated
soil by a trenching machine. The horizontally oriented strip
heaters may be covered with the cuttings made during the formation
of the trenches. The cuttings may be tamped down on top of the
strip heaters. Horizontal strip heaters may be less expensive to
install than are vertical strip heaters. Trenching costs are
generally less than drilling costs. Also, horizontally positioned
strip heaters may be very long. Rows of strip heaters may be
separated by distances equal to about twice the insertion depth of
the strip heaters into the soil.
The heater section of a strip heater and the power source are
designed to supply heat input into the soil that is greater than
the heat input that the soil can absorb, but not enough to overheat
the strip heater. An average soil may be able to absorb about 300
W/ft. A strip heater may be designed to have a maximum heat input
of about 600 W/ft. The temperature that a strip heater attains is
self-regulating. As the temperature of a strip heater increases,
the resistance of the strip heater increases. The power source
provides a substantially constant voltage to the strip heaters, so
an increase in the resistance of a strip heater decreases the power
dissipation of the strip heater. The application of a steady
voltage to a series of heater strips may result in steady state
power dissipation through the strip heaters. Heater sections of
strip heaters may be sized to allow the strip heaters to attain
temperatures up to about 2000.degree. F. when energized by a power
source. The strip heaters may be designed to operate at about
1600.degree. F. A 304 stainless steel strip heater may have a
resistance of about 0.08 ohms at about 1600.degree. F.
The strip heaters may be directly connected by copper cable to a
power source. The power source may be a transformer. A group of
strip heaters may be connected in series to the transformer. The
strip heaters may be directly connected to the transformer without
well controllers or silicon controlled rectifiers.
The simple geometry of a strip heater may allow a strip heater to
be produced at a cost of about $1.8 per foot. The production cost
of a strip heater may result in about a 100.times.cost reduction as
compared to the production of a conventional heater well. The
production cost for a heater strip and suction well may be about $5
per foot. The production cost of a heater strip and suction well
may result in about a 50.times.cost reduction as compared to the
production of a conventional heater/suction well. Heater strips and
heater strip and suction well combinations may not require external
casings like conventional heater wells and heater/suction
wells.
Installation costs of a heater strip in an augered hole may be
greatly reduced. A conventional heater well took approximately 6
hours of time to both install in an augered hole and connect to a
power supply. A strip heater may take 10 minutes or less to install
and connect to a power supply. Installation costs of installing a
heater strip and suction well combination may also be greatly
reduced as compared to installing a conventional heater/suction
well.
A collection system may connect all of the suction wells of a soil
remediation system to a treatment facility. The collection system
may include hoses and a vacuum manifold. The hoses may be high
temperature hoses. The hose may be, but is not limited to, a high
temperature rubber hose, a high temperature silicone rubber hose,
or a coated rubber flexible metal hose. The system operates under
vacuum; therefore, the hose needs to have structural strength that
inhibits collapse of the hose. The hose may be a double walled hose
or a steel reinforced hose. The vacuum manifold may be plastic
piping, such as chlorinated polyvinyl chloride piping. Off-gas
passing through a hose has a residence time within the hose due to
the length of the hose. The residence time may be sufficiently long
to allow the off-gas to cool to a temperature within the working
temperature limits of the vacuum manifold piping. A hose may be
from about 4-feet to over 40-feet in length.
The use of a hose and plastic piping collection system results in
lower costs, simplified on-site construction, and lower
mobilization costs as compared to a conventional metal piping
collection system. The collection system is not insulated and
heated to prevent condensation of the off-gas. This greatly reduces
the cost, installation time, and operating cost of the collection
system. The hose may be rolled into coils for transportation.
Plastic piping may be purchased locally near the site. Hose and
plastic piping are easily cut to size on-site and are connectable
by solvent gluing. The need to have precise positioning of metal
pipes is eliminated. Also, hose and plastic piping are lightweight
and do not require machinery to lift and position during
installation. For soil contaminated with chlorinated compounds, the
off-gas may contain significant amounts of HCl. Unlike metal
piping, hose and the plastic piping may be highly resistant to
corrosion caused by the off-gas.
A treatment facility processes off-gas from the soil to
substantially remove contaminants within the off-gas. A treatment
facility may also provide vacuum that removes the off-gas from the
soil. The treatment facility may include a condenser that separates
the off-gas into a liquid stream and a vapor stream. The liquid
stream and the vapor stream may be separately processed to remove
contaminants. The liquid stream may be treated using a separator
and an activated carbon bed. The vapor stream may be treated using
an activated carbon bed or an air stripper.
The treatment facility does not require the use of a thermal
oxidizer as did previous treatment facilities. Removing the thermal
oxidizer from the treatment facility eliminates the large capital
cost, transportation costs, and operating expenses associated with
the thermal oxidizer. The elimination of the thermal oxidizer may
allow the soil remediation process to be run unattended. A site
supervisor may periodically check the system and perform normal
maintenance functions at the site to ensure proper operation of the
soil remediation system. Continuous manned operation of the in situ
soil remediation process may not be required.
BRIEF DESCRIPTION OF THE DRAWINGS
Other advantages of the invention will become apparent upon reading
the following detailed description and upon reference to the
accompanying drawings in which:
FIG. 1 shows a representation of a heater strip vertically inserted
into soil;
FIG. 2 shows a representation of a heater strip horizontally
inserted into soil;
FIG. 3 is a perspective view of a portion of a heater section that
has a varying cross sectional area;
FIG. 4 shows a representation of a heater strip positioned within
an external casing;
FIG. 5a shows a side representation of a heater/suction well
inserted into soil;
FIG. 5b shows a front representation of a heater/suction well
inserted into soil;
FIG. 6 shows a representation of an in situ thermal desorption
system using heater/suction wells;
FIG. 7 shows a diagrammatic representation of a treatment
facility;
FIG. 8 shows a representation of a square well pattern layout using
alternating heater wells and heater/suction wells;
FIG. 9 shows a representation of a triangular well pattern
layout;
FIG. 10 shows a representation of a horizontal strip heater
placement pattern;
FIG. 11 shows voltage, amperage, and power data for a strip heater
as a function of time;
FIG. 12 shows temperature data for the strip heater of FIG. 11 as a
function of time;
FIG. 13 shows resistance data for the strip heater of FIG. 11 as a
function of time; and
FIG. 14 shows potential voltage near the strip heater of FIG. 11 as
a function of time.
While the invention is susceptible to various modifications and
alternative forms, specific embodiments thereof are shown by way of
example in the drawings and will herein be described in detail. It
should be understood, however, that the drawing and detailed
description thereto are not intended to limit the invention to the
particular form disclosed, but on the contrary, the intention is to
cover all modifications, equivalents and alternatives falling
within the spirit and scope of the present invention as defined by
the appended claims.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
As depicted in FIG. 1, an in situ thermal desorption soil
remediation process may use strip heaters 20 to conductively apply
heat to contaminated region 22 of soil 24. The soil remediation
process may be used to remove contaminants from the soil 24. A
vacuum may be applied to the soil 24 along with conductive heat to
remove contaminants from the soil. FIG. 1 shows an embodiment of a
strip heater 20 vertically positioned in contaminated soil 24. FIG.
2 shows an embodiment of a strip heater 20 horizontally positioned
in soil 24. Strip heaters 20 may also be inserted in soil 24 in
non-vertical and non-horizontal (e.g., angled) orientations.
A strip heater 20 may include heater section 26, transition
sections 28, and pins 30. The heater section 26 may be formed of a
high temperature, chemical resistant metal. The heater section 26
may be formed of a stainless steel, including but not limited to
type 304 stainless steel, type 309 stainless steel, type 310
stainless steel, or type 316 stainless steel. Heater sections 26
may also be formed of other metals including, but not limited to,
nichrome.RTM., incoloy.RTM., hastelloy.RTM., or monel.RTM.. For
example, the heater section 26 may be made of nichrome.RTM. 80 or
incoloy.RTM. 800.
The specific metal used to form the heater section 26 may be chosen
based on cost, the operative temperature of the soil remediation
process, the electrical properties of the metal, the physical
properties of the metal, and the chemical resistance properties of
the metal. For example, 310 stainless steel is a high temperature
stainless steel that may dissipate about 20% more power than 304
stainless steel for strips of equivalent dimensions. The corrosion
resistance of 310 stainless steel is better than the corrosion
resistance of 304 stainless steel. The upper working temperature
limit of 310 stainless steel is about 300.degree. F. higher than
the upper working temperature limit of 304 stainless. The extra
temperature range of 310 stainless steel may be used to dissipate
extra heat into soil 24 and shorten remediation time. The extra
temperature range may be used as a safety margin to insure against
overheating the strip heater 20. The cost of 310 stainless steel
may be about 25% more than the cost of 304 stainless steel. At a
design stage of a soil remediation process, a determination may be
made of whether the better characteristics of 310 stainless steel
justify the extra cost of the 310 stainless steel above the cost of
304 stainless steel.
A heater section 26 may have a large cross sectional area as
compared to conventional wire heater elements. A large cross
sectional area of a heater section 26 allows a strip heater 20 to
have a small electrical resistance as compared to a conventional
heater of equivalent length. A small electrical resistance allows a
strip heater 20 to be long. A small electrical resistance also
allows several strip heaters 20 to be electrically connected in
series. The ability to connect several strip heaters 20 in series
may greatly simplify the wiring requirements of a soil remediation
system. The large cross sectional area of a heater section 26 may
provide a large contact area between the heater section and
material placed adjacent to the strip heater 20. The large contact
area may provide good thermal contact between the heater section 26
and the adjacent material. Good thermal contact may promote
dissipation of heat produced in the heater section 26 into
surrounding soil 24.
A heater section 26 may have a substantially rectangular cross
sectional area. For example, an embodiment of a heater section 26
may have a 1-inch by 1/8-inch rectangular cross section and a
length of about 20 feet. Cross sectional shapes other than
rectangular shapes may also be used. The cross sectional shape may
be, but is not limited to ellipsoidal, circular, arcuate, or square
shapes. The cross sectional area and length of a heater section 26
may be designed to allow the temperature of the heater section to
reach a desired temperature when power is applied to the strip
heater 20.
A heater section 26 may be formed with variable cross sectional
areas located at positions along a length of the heater section.
Greater heat dissipation will occur at portions of a heater section
26 that have less cross sectional area, and less heat dissipation
will occur at portions of the heater section that have greater
cross sectional area. Local high heat dissipation portions of a
heater section 26 may be positioned adjacent to sections of soil 24
that require extra heating, such as wet soil areas. Low heat
dissipation areas of a heater section 26 may be positioned adjacent
to sections of soil 24 that do not require heating, such as
impermeable soil layers. Various portions of metal having different
cross sectional areas may be welded together to form a heater
section 26 having variable cross sectional areas. FIG. 3 shows a
portion of a heater section 26 that has reduced area 32 that
dissipates heat at a greater rate than adjacent areas 34.
Transition sections 28 may be welded to each end of a heater
section 26. Pins 30 may be welded to the transition sections 28.
For example, the transition sections 28 shown in FIG. 1 may be
6-inch long strips of 1-inch by 1/2-inch stainless steel that are
welded to the ends of a 1-inch by 1/8-inch heater section 26. The
pins 30 may be 3/8-inch diameter nickel pins. Pins 30 may extend
above soil surface 36 when the strip heater 20 is inserted into the
soil 24. Lugs 38 may be used to splice the pins 30 to cable 40. The
cable 40 may be one conductor, 3/0 gauge, type "G" mining cable.
The cable 40 may be electrically coupled to power source 42. The
power source 42 may be a transformer. Long strips (not shown) of
low resistance material, such as nickel, may be welded to the
heater section 26 to form long unheated sections of a strip heater
20. Long unheated sections of a strip heater 20 may be needed to
position heater section 26 next to contaminated soil 44 that is not
near the soil surface 36.
A strip heater 20 is positioned in the soil 24. A portion of the
heater section 26 may be positioned below contaminated soil 44 so
that a layer of uncontaminated soil 46 is heated during a soil
remediation process. The portion of the heater section 26 below the
contaminated soil 44 may be one or more feet in depth. Heating a
section of uncontaminated soil 46 below the contaminated soil 44
may prevent fall off in temperature at the boundaries of the
contaminated region. The cross sectional area of the strip heater
20 at the bottom of the well 50 may be small so that more heat is
diffused in the bottom of the strip heater. Diffusing more heat at
the bottom may help to establish a more uniform temperature
distribution throughout the soil treatment area.
A strip heater 20 that is not horizontally positioned in the soil
24 may be formed in a U-shape so that both strip heater pins 30 are
accessible near the soil surface 36, as depicted in FIG. 2. A strip
heater 20 may be inserted in opening 47 within the soil 24. In an
embodiment, the opening 47 may be an augered hole. As an
alternative to inserting a strip heater 20 in an augered hole, the
strip heater may be driven into the soil 24. A drive rod (not
shown) may be positioned at the center of a strip heater 20. The
drive rod may then be pounded into the soil 24. When the strip
heater 20 is inserted to a desired depth, the drive rod may be
withdrawn. The drive rod does not need to be a continuous rod. The
drive rod may be made of threaded sections that are assembled
together as the drive rod is pounded deeper into the soil 24. A
geoprobe or a cone penetrometer rig may be used to drive a strip
heater 20 into the soil 24. Also, a sonic rig (not shown) may be
used to vibrate a strip heater 20 to a desired depth. The sonic rig
may include an eccentric cam that vibrates a strip heater 20 and a
drive rod to a desired soil depth.
Driving or vibrating a strip heater 20 into soil 24 may eliminate
problems associated with disposing of cuttings formed during the
formation of an augered hole. Avoidance of the production of
cuttings may be particularly advantageous at extremely toxic or
radioactive sites. Also, driving or vibrating a strip heater 20
into the soil 24 advantageously places a portion of a strip heater
20 in direct contact with the soil to be heated.
After a strip heater 20 is inserted into the soil 24, fill material
48 may be placed adjacent to the strip heater 20 to form heater
well 50. To place the fill material 48 against the heater strip 20,
a pipe (not shown), such as a polyvinyl chloride pipe, may be
inserted between legs 52 of a U-shaped strip heater 20. The pipe
may press the strip heater 20 against the soil 24. Fill material 48
may be inserted through the pipe while raising the pipe out of the
soil 24. The fill material 48 may press the strip heater 20 against
the soil 24. The fill material 48 may also plug spaces between the
strip heater 20 and the soil 24. The fill material 48 may include
sand and/or gravel. The fill material 48 may also include catalyst
material, such as aluminum oxide. The catalyst material may promote
the thermal degradation of contaminants that pass through the fill
material 48. The fill material 48 may be built up in a mound at the
soil surface 36 to promote water runoff away from the heater well
50. Thermocouple well 54 may be positioned in the fill material 48
between the legs 52 of a U-shaped strip heater 20. A thermocouple
placed in the thermocouple well 54 may be used to measure the
temperature between the legs of the strip heater during an in situ
thermal desorption soil remediation process.
FIG. 4 shows an embodiment of a heater well 50 wherein a strip
heater 20 is placed inside an external casing 56. The external
casing 56 inhibits contact of the strip heater 20 with formation
fluids within soil 24. The external casing 56 may be a carbon steel
pipe. The strip heater 20 may be spaced away from a wall of the
external casing 56 by insulated centralizers 58 positioned on the
wall of the casing 56 or on the strip heater 20. The casing 56 may
be packed with heat conductive fill material 48. The external
casing 56 may be in thermal contact with adjacent soil 24. Selected
strip heaters 20 of a soil remediation system may be placed in
external casings 56 to reduce current leakage into the soil 24 from
the strip heaters. Some current leakage may be acceptable because
the current leakage may heat water or soil drawing current from the
strip heaters 20. If excessive current leak is possible, an
external casing may be used to surround the strip heater. An
external casing 56 may be used when the heater well 50 is inserted
in a water saturated zone, or into a brackish water. An external
casing 56 may add an additional cost of about $3 per foot for a
heater well 50.
A heater well 50 may include vacuum casing 60 positioned between
the legs 52 of a heater strip 26 and separated from the heater
strip by insulated centralizers 58, as depicted in FIG. 5a. A
portion of the casing 60 that is positioned adjacent to
contaminated soil 44 may be perforated. Perforations allow a vacuum
to draw vapor into the casing 60 so that the vapor may be removed
as off-gas from the soil 24. Strip heater 20 and casing 60
combinations form heater/suction wells 62. FIGS. 5a and 5b shows an
embodiment of a heater/suction well 62. The fill material 48
adjacent to the casing 60 of a heater/suction well 62 may include
catalyst material, such as aluminum oxide, that enhances the
thermal degradation of contaminants as the contaminants pass
through the fill material into the casing. As shown in FIGS. 5a and
5b, thermocouple well 54 may be inserted into the casing 60. In an
embodiment, the thermocouple well 54 is 1/4-inch stainless steel
tubing that-is inserted into the center of a 1.5-inch stainless
steel Casing 60. A thermocouple positioned within the thermocouple
well 54 may be used to monitor the temperature of a strip heater 20
adjacent to the casing 60.
A strip heater 20 of a heater/suction well 62 may be placed inside
an external casing 56. A vacuum casing 60 may be strapped to the
outside of the external casing 56 and strip heater 20 combination
shown in FIG. 4. Alternately, a large diameter vacuum casing 60 may
be placed on the outside of the external casing 56 and strip heater
20 combination shown in FIG. 4. An annular space may be formed
between the vacuum casing 60 and the external casing 56 containing
the strip heater 20.
FIG. 6 shows a representation of an embodiment of an in situ
thermal desorption soil remediation system that uses a plurality of
vertical heater/suction wells 62 to treat contaminated region 22 of
soil 24. The soil remediation system may include the heater/suction
wells 62, impermeable barrier 64, power source 42, collection
piping, and treatment facility 66. The collection piping may
include hoses 68 and vacuum manifold 70. The soil remediation
process may involve simultaneously applying heat and vacuum to the
soil 24. Heat may be applied to the soil 24 by electrical powered
strip heaters 20 positioned within the heater/suction wells 62. The
heat applied by the strip heaters 20 to the soil 24 may destroy
some soil contamination by thermal degradation of the contaminants.
The vacuum may draw some contaminants out of the soil 24 as
components of off-gas removed from the soil. The off-gas may travel
through the hoses 68 and the vacuum manifold 70 to the treatment
facility 66.
Impermeable barrier 64 may be placed over the remediation site. The
barrier 64 may be silicone rubber sheeting. If the contaminated
soil 44 is not near the soil surface 36, and if the strip heaters
20 are placed some distance below the soil surface, it may be
possible to avoid high surface temperatures. If the soil surface 36
temperature is low, then a low temperature material such as
polypropylene sheeting or polyethylene sheeting may be used as the
impermeable barrier 64. Sealant 72 may be applied around each
portion of a well 62 that extends through the barrier 64. Strip
heater pins 30 and vacuum casings 60 may extend through the barrier
64. The barrier 64 may limit the amount of air drawn into the soil
24 during a soil remediation process.
Thermal barrier 74 may be placed under or on top of an impermeable
barrier 64. FIG. 1 shows a thermal barrier 74 placed under an
impermeable barrier 64. The thermal barrier 74 may inhibit heat
loss from the soil surface 36. The thermal barrier 74 may inhibit
melting and thermal degradation of the impermeable barrier 64. The
thermal barrier 74 may also inhibit vapor that seeps out of the
soil 24 from condensing on the impermeable barrier 64. The thermal
barrier 74 may be mineral or cotton wool, glass wool or fiberglass,
polystyrene foam, or aluminized mylar.
A power source 42 used to heat the strip heaters 20 may be a three
phase transformer. For example, the power source 42 may be a 112.5
kVA transformer that has a 480 VAC 3-phase primary and a 208 VAC
3-phase secondary. Each phase may be used to power a group of strip
heaters 20 that are electrically connected in series. If more than
three groups of strip heaters 20 are needed to treat a soil
contamination region 22, sections of the contamination region may
be sequentially treated, or additional power sources may be used to
treat the entire region at one time. The strip heaters 20 may be
directly coupled to the power source 42 without the use of well
controllers or silicon controlled rectifiers. The strip heaters 20
and the power source 42 are designed so that the strip heaters heat
to a desired temperature when connected to the power source. The
strip heaters 20 may be designed to heat to a temperature that is
about 2000.degree. F. The strip heaters may be designed to have a
steady state operating temperature of about 1600.degree. F.
Off-gas drawn from the soil 24 by vacuum may pass through the hoses
68 and the vacuum manifold 70 to the treatment facility 66. The
hoses 68 may attach to vacuum casings 60 of a well 62 and to the
vacuum manifold 70. The casing 60 may extend through the
impermeable barrier 64. Sealant 72, such as silicone rubber
sealant, may be used to seal casings 60 to the barrier 64 to
preserve the integrity of the barrier.
A hose 68 may be attached to each vacuum casing 60 and to the
vacuum manifold 70 by solvent glue and/or clamps (not shown), or by
other attachment methods including, but not limited to, threading
or flanges. The hoses 68 may be formed of high temperature rubber
that has an upper working temperature limit of about 450.degree. F.
The hoses 68 are conduits for transporting off-gas from the casings
60 to the vacuum manifold 70. Off-gas passing through a hose 68 has
a residence time within the hose. A hose 68 may have a sufficient
length so that the residence time of off-gas within the hose is
sufficiently long to allow the off-gas to cool. The off-gas may
cool within the hoses 68 to a temperature that is at or below an
upper working temperature limit of the material that forms the
vacuum manifold 70.
A vacuum manifold 70 may be formed of plastic piping. The plastic
piping may be chlorinated polyvinyl chloride (CPVC) piping or other
plastic piping that has a high upper working temperature limit. The
upper working temperature limit of CPVC pipe is approximately
200.degree. F. Off-gas flowing through the vacuum manifold 70 may
continue to cool. Portions of the vacuum manifold 70 located away
from the vacuum casings 60 may be formed of plastic piping, such as
PVC piping, that has a lower working temperature limit than CPVC
piping.
The use of a collection system including hoses 68 and plastic
piping vacuum manifold 70 may result in lower costs, simplified
on-site construction, and lower mobilization costs as compared to a
metal piping collection system. The collection system is not
insulated and heated to prevent condensation of the off-gas. This
greatly reduces the cost, installation time, and operating cost of
the collection system. The hose 68 may be rolled into coils for
transportation. Plastic piping may be purchased locally near the
site. Hose 68 and plastic piping are easily cut to size on-site and
are connectable by solvent gluing. Also, hose 68 and plastic piping
are lightweight and do not require machinery to lift and position
during installation. Unlike some metal piping, hose 68 and the
plastic piping may be highly resistant to corrosion caused by the
off-gas.
FIG. 7 shows a representation of an embodiment of a treatment
facility 66. Off-gas from the vacuum manifold 70 may pass into
separator 76. The separator 76 may separate the off-gas into a
liquid stream and a vapor stream. Vacuum system 78 that is in-line
with the vapor stream may provide the vacuum to the soil 24 that
removes off-gas from the soil. The vacuum system 78 should be
capable of pulling a vacuum appropriate for the particular
combination of soil permeability and geometry of the wells. The
vacuum system 78 may be able to pull a vacuum in the range of 0.1
to 14.6 psi. The vacuum pump may be a water-sealed vacuum pump.
The liquid stream and the vapor stream may be processed to reduce
contaminants within the streams to acceptable levels. Monitoring
equipment (not shown) installed in the treatment facility 66 may
determine the quantity of contaminants in the processed streams.
The monitoring equipment may sound an alarm if too much
contamination is discharged from the treatment facility 66.
The liquid stream may be separated by second separator 80 into a
non-aqueous stream and an aqueous stream. In an embodiment, the
second separator 80 and the separator 76 may physically be a single
unit. The non-aqueous stream may include oils and other non-aqueous
material. The non-aqueous stream may be very small compared to the
aqueous stream. The non-aqueous stream may be sent to treatment
unit 82. The treatment unit 82 may place the non-aqueous stream in
storage containers, such as waste barrels. The containers may be
transported off-site for disposal. Alternately, the treatment unit
82 may be an oxidization system that destroys the non-aqueous
stream.
Pump 84 may transport the aqueous stream through activated carbon
bed 86. The activated carbon bed 86 removes contaminants from the
aqueous stream. The remaining aqueous stream may then be
discharged. For example, after passing the aqueous stream through
the activated carbon bed 86, the aqueous stream may be sent to
sanitary sewer 88.
The vapor stream from the separator 76 may pass through treatment
unit 90. The treatment unit 90 may be an activated carbon bed. The
activated carbon bed removes contaminants from the vapor stream.
Alternately, the treatment unit 90 may be a stripper that removes
contaminants from the vapor stream. Blower 92 may draw the vapor
stream through the treatment unit 90 and vent the remaining vapor
to the atmosphere.
The wells shown in FIG. 1 are heater/suction wells 62. The
heater/suction wells 62 may be used to both heat the soil 24 and
remove off-gas from the soil. The off-gas may be removed from the
soil by vacuum produced by the treatment facility 66. In alternate
embodiments, a well may be only a heater well 50. FIG. 8 shows a
square well placement pattern wherein the wells are alternately
heater/suction wells 62 and heater wells 50. Other embodiments may
use other ratios of heater wells 50 to heater/suction wells 62. In
other embodiments, some wells may be only suction wells (not
shown).
The wells shown in FIGS. 6 and 8 are placed in a square pattern.
Other regular or irregular well positioning patterns may be used to
meet the particular needs of a soil remediation site. Examples of
other regular patterns that may be used include triangular
patterns, rectangular patterns, or hexagonal patterns. For example,
wells may be placed in a triangular pattern as shown in FIG. 9. The
outer wells and the center well may be heater wells 50 and the
other wells may be heater/suction wells 62. Triangular and
hexagonal well patterns may closely conform to circular or
oval-shaped contamination areas. Square and rectangular patterns
may closely conform to property lines. Irregular patterns may be
used to avoid underground obstructions, to concentrate several
wells in a highly contaminated soil region, or to meet other
particular needs of a soil remediation system.
FIG. 10 shows a horizontal layout pattern for three strip heaters
20. A horizontally positioned strip heater 20 may be placed in a
trench that was previously formed by a trenching machine. After the
strip heaters 20 are positioned within trenches and electrically
coupled a power source 42, the cuttings formed when making the
trench may be tamped down on top of the strip heaters. Horizontally
positioned strip heaters 20 may be used to treat low depth soil
contamination that is within about 4 feet of the soil surface 36.
Horizontally positioned strip heaters 20 may be have long lengths
that span across a contaminated region 22 of soil 24. Rows of strip
heaters 20 may be separated by distances equal to about twice the
insertion depth of the strip heaters into the soil 24.
As shown in FIG. 2, a horizontally positioned strip heaters 20 may
be placed below contaminated soil 44. Vacuum applied to the soil 24
by a treatment facility 66 may be applied near the soil surface 36.
Permeable mat 94 may be placed on top of the soil surface 36, and
an impermeable barrier 64 and a thermal barrier 74 may be placed on
top of the mat. The mat 94 may serve as a conduit for flow beneath
the impermeable barrier 64. In an embodiment, the mat 94 may be a
thin layer of high permeability sand or other granular material.
The mat 94 may include catalyst material that enhances thermal
degradation of contaminants that pass through the mat. The mat 94
may allow off-gas to flow out of the soil 24 to a vacuum manifold
70 positioned above the mat. The off-gas may flow even when the
vacuum draws the impermeable barrier 64 against the mat and
compresses the mat. Alternately, suction wells (not shown) may be
inserted into the soil 24 throughout the treatment site to draw
off-gas from the soil. The suction wells may be coupled to hoses
68, a vacuum manifold 70, and a treatment facility 66.
To implement an in situ thermal desorption soil remediation
process, strip heaters 20 may be placed in the soil 24 at the
remediation site. For low depth soil contamination, the strip
heaters 20 may be horizontally positioned in the soil 24. For
deeper soil contamination, the strip heaters 20 may be vertically
positioned in the soil 24. Suction wells may be positioned
throughout the soil remediation site. Some or all of the suction
wells may be coincident to the strip heaters 20.
A thermal barrier 74 may be positioned over the remediation site.
Suction well casings 60 and the strip heater pins 30 may be pushed
through the thermal barrier 74. An impermeable barrier 64 may be
positioned over the thermal barrier 74. Suction well casings 60 and
strip heater pins 30 may be pushed through the impermeable barrier
64. Sealant 72 may be applied to each break in the impermeable
barrier 64 to seal the barrier to the strip heater pins 30 and to
the suction well casings 60.
The suction well casings 60 may be attached to a vacuum system 78
of a treatment facility 66. The vacuum system 78 may be engaged to
begin removing off-gas from the soil 24. Off-gas from the soil 24
may be treated by the treatment facility 66 to reduce contaminants
within the off-gas to acceptable levels.
Groups of strip heaters 20 may be electrically connected together
in series. Each group of strip heaters 20 may be connected to a
power source 42. When the strip heaters 20 are connected to a power
source 42, the power supplied to the strip heaters 20 will heat the
heater sections 26 of the strip heaters 20. The heat may
conductively transfer to the soil 24. The heat may enhance removal
of contaminants from the soil 24. The soil remediation system may
be run for months or longer.
Treatability studies of laboratory soil samples indicate that
removal of contaminants from soil may be a function of both
temperature and time. A long operation time favors removal of
contaminants that may have time dependent thermal desorption
mechanisms. Heating times for natural soils may be longer than
vaporization or desorption times for contaminants. Thus, complete
removal of contaminants may be effected if the soil is heated to an
adequate temperature. The adequate temperature may be the boiling
point of the contaminant. For partly water soluble contaminants, an
ultrahigh remediation goal may only be obtainable if pore water
within the soil is boiled.
FIG. 11 shows electrical data for a strip heater 20 positioned
within a heater well 50 for an eleven day test. The power started
at about 500 W/ft and decreased to a steady state value of about
380 W/ft after about 20 hours of operation. The decrease in the
power reflects heating and drying of the sand and soil adjacent to
the strip heater 20. FIG. 12 shows the temperature rise of the
strip heater 20 as measured by a thermocouple positioned between
legs 52 of the strip heater. FIG. 13 shows the calculated increase
in resistance of the strip heater 20 as a function of time. As the
resistance increases over the first 20 hours, the power dissipation
of the heater section 26 decreases. An increase in resistance may
also be a result of corrosion metal loss. However, very little
corrosion was observed after more than 10 days of operation. FIG.
14 shows values of ground potential measured at copper stakes
located 7-inches and 14-inches away from the heater. The initial
ground potential was only 0.5 V, and it decreased to a background
level of 40 mV after about 20 hours of operation when the sand and
soil dried out. The dry sand and soil were excellent electrical
insulators.
Further modifications and alternative embodiments of various
aspects of the invention will be apparent to those skilled in the
art in view of this description. Accordingly, this description is
to be construed as illustrative only and is for the purpose of
teaching those skilled in the art the general manner of carrying
out the invention. It is to be understood that the forms of the
invention shown and described herein are to be taken as examples of
embodiments. Elements and materials may be substituted for those
illustrated and described herein, parts and processes may be
reversed, and certain features of the invention may be utilized
independently, all as would be apparent to one skilled in the art
after having the benefit of this description of the invention.
Changes may be made in the elements described herein without
departing from the spirit and scope of the invention as described
in the following claims.
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